LastEffect

There is 3 types of more or less accurate spell effect you can expect by casting:

Tooltip data from Blizzard (seems always lower than actual effect)

Heavy calculations and tuning like DrDamage do

Collect data and show (like this addon)

For now supports only Blizzard's action buttons and Bartender4 since v0.20.

How it works

Lot of spells quite plain: you pressing button, it have spell id, through combat log addon receives damage/healing info. For example you casting "Smite" - log contains spell cast started and damage info (effect will be shown on next spell cast).
Some spells more complicated: you casting instant spell "Icy Touch", it contains cast success, direct damage info, "Frost Fever" aura applied that causing extra damage per tick. Some of such spells solved already, some not (I have only several 80+ alts). So if you want to get more info read next section "Howto".

Howto

To add damage/heal caused by spell's side effects:

Find "Target Dummy"

Enable debug mode like "/le debug"

Cast your spell and see log in chat

There will be cast and auras info (format is "event, spellId, spellName, amount or buff"), add your side effect spellId to LE_SpellPartOf in Spells.lua like [sideEffectId] = mainSpellId

Hello - unfortunately it seems like none of my spells are there by default, so Im having to add them all. I'm having trouble understanding how do this. Specifically the "sideEffectId" term from your How-To are confusing me, I cant figure out where you find that ("[sideEffectId] = mainSpellId"). I tried looking at the previous comments about Fire Nova but he's got so much going on in that log it confuses me.

None of spells shown? I have elemental shammy and at least "Lightning Bolt" works by itself (spell id for damage is same that casting spell id). Are you using standard Blizzard's bars or something based on this bars (like Bartender or Dominos)?

"How to" is simple, you have in your log for example "SPELL_CAST_START, 51505, Lava Burst" (spell id 51505). "Side effects" for example if you casting spell with spell id X and have damage from spell id Y (damage from applied dots for example). Since addon can't figure such things you should assign in Spells.lua "[Y] = X, --can comment here" to add damage from effect Y to casted spell X.

Slightly more complicated things like casting "Icy Touch" and adding damage from "Frost Fever" (but there is spell tooltip, "Icy Touch blasts ... and apply ..."). Except in case of DK many things applying "Frost Fever" and it is reasonable to set only one spell as "Frost Fever" damage collector.

Hello - unfortunately it seems like none of my spells are there by default, so Im having to add them all. I'm having trouble understanding how do this. Specifically the "sideEffectId" term from your How-To are confusing me, I cant figure out where you find that ("[sideEffectId] = mainSpellId"). I tried looking at the previous comments about Fire Nova but he's got so much going on in that log it confuses me.

... couple of things I noticed quickly on my enhance shammy, the buttons for Storm Strike and fire nova are not registering ...

Well, my shaman elemental/resto and only 46. But adding some spells quite simple: by activating debug mode you will see log in chat as "event, spellId, spell name, amount/buff". So if some spell after cast creates another aura by id - you should add to Spell.lua LE_SpellPartOf table like "[additional effect id] = casted spell id,". For example dk's "Icy Touch" (id 45477) causing "Frost Fever" (id 55095), so it is "[55095] = 45477," in those table. You should look for DAMAGE and HEAL events.
Post your data here and I will add this to Spell.lua for others.

Addon logic changed to amount of damage/heal between button clicks. It helps to solve issue with channeled spells that looks like several independent casts in combat log. Also that required to make average mode.
Added optional "average" mode (see slash commands: type "/le" or "/LastEffect" in chat to see available). This mode suppose to show average of last 10 casts.

Possible issue: there is time between click and actual damage (depends from distance) and by casting nonstop sometime 2 casts can be count as 1. Can't find solution yet.

at lines 328-333 in main.lua v0.21 (use editor like Notepad++, replace commented text there by text above). Confirm is it works or throwing errors. I'm not ElvUI user and not trying to use it - ElvUI changes global variables and I have headache fixing my chat window etc.

... couple of things I noticed quickly on my enhance shammy, the buttons for Storm Strike and fire nova are not registering ...

Well, my shaman elemental/resto and only 46. But adding some spells quite simple: by activating debug mode you will see log in chat as "event, spellId, spell name, amount/buff". So if some spell after cast creates another aura by id - you should add to Spell.lua LE_SpellPartOf table like "[additional effect id] = casted spell id,". For example dk's "Icy Touch" (id 45477) causing "Frost Fever" (id 55095), so it is "[55095] = 45477," in those table. You should look for DAMAGE and HEAL events.
Post your data here and I will add this to Spell.lua for others.

You just became my favorite person!, since DR damage is no more I was really missing this, couple of things I noticed quickly on my enhance shammy, the buttons for Storm Strike and fire nova are not registering, will test some more on some alts later once again Thank you !